When does the fragmentation happen at L3?

[Iluvnetwork]

If MSS is calculated without any errors, fragmentation cannot happen at L3. Plus, I recently heard that fragmentation does not happen for TCP & UDP. Then, when does the fragmentation happen at L3? 

Lastly, if a segment gets fragmented at L3, only first one has a L4 header and others don't. In this case, what would happen?

[Joseph W. Doherty · Accepted solution]

"If MSS is calculated without any errors, fragmentation cannot happen at L3."

Incorrect.

"I recently heard that fragmentation does not happen for TCP & UDP."

Incorrect.

"Then, when does the fragmentation happen at L3?"

It happens when a received TU is larger than the L3 egress MTU and DF-bit is not set.

"Lastly, if a segment gets fragmented at L3, only first one has a L4 header and others don't. In this case, what would happen?"

Receiver will reassemble the fragments. (Of course if all the fragments are not received, the reassembly cannot be completed and the L3 packet is "lost".)